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1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to a system that compensates for polarization mode dispersion in an optic fiber.
2. Description of the Prior Art
In fiber optic communication systems, a fiber that carries optical signals contains asymmetries. These asymmetries result in the optical properties of the fiber not being the same in all directions. Thus, the fiber is birefringent, where the material displays two different indices of refraction. This fiber birefringence causes polarization mode dispersion (PMD).
PMD is measured like a vector quantity, where a differential group delay is the magnitude of the vector and the principal state of polarization (PSP) are the direction. There are two PSPs associated with PMD. The two PSPs propagate at slightly different velocities with the distribution of signal power varying with time. PMD is a time varying stochastic effect. PMD varies in time with ambient temperature, fiber movement, and mechanical stress on the fibers. Compensating for PMD can be difficult because of the time varying nature and randomness of PMD.
Prior systems that involve taking the fiber out of operation to compensate for PMD are expensive. There have been few systems that have attempted to compensate for PMD on active fibers. A fiber is active when the fiber is operational to exchange user information. One prior system uses a polarization controller at the transmitter. The polarization controller aligns the input state of polarization of the input optical signal to the PSP of the fiber to reduce the signal distortion. One disadvantage of this system is the requirement of timely knowledge of the PSPs, which is difficult at best. Another disadvantage is the PSP of the fibers are different for each receiver. When optical add/drops are involved, this system is ineffective.
Another system uses a polarization controller prior to the receiver. The polarization controller aligns the polarization of one of the PSPs with a polarization filter. The polarization controller also receives control signals from a feedback arrangement. This system processes one of the PSPs which is essentially free from the PMD effects.
Another system uses a polarization controller and a length of polarization-maintaining fiber prior to the receiver. The length of the polarization-maintaining fiber is selected so a fixed value of differential group delay is equal to the average differential group delay of the long fiber to minimize the PMD effects. A disadvantage is this system only works for a fixed value of differential group delay. When differential group delay varies, the system does not fully compensate for the PMD effects.
Another system monitors the effect of PMD on an input optical signal. The power level of a non-return-to-zero (NRZ) optical signal""s spectral component corresponding to one-half of the data rate indicates the PMD in a fiber link. In one example, to monitor the PMD on a 10 Gb/s NRZ optical signal, the system monitors the power of the spectral component at 5 GHz. This system comprises a narrowband filter centered at 5 GHz followed by a square-law detector and a lowpass filter.
One problem is that none of the prior systems track changes in the differential group delay, which is a component of PMD. Another problem is the degraded ability to monitor for DGD and PSPs when the input state of polarization of the input signal is nearly aligned with one of the PSPs. A system is needed that can compensate for PMD which accounts for changes in the PMD and the problems when the input state of polarization of the input signal is nearly aligned with one of the PSPs.
The invention solves the above problems by compensating for PMD. A polarization scrambler scrambles a state of polarization of an optical signal that carries user information. A PMD compensation system then receives the optical signal over an active optic fiber. The PMD compensation system then measures a differential group delay and principal states of polarization of the polarization mode dispersion in the active optic fiber. The PMD compensation system then determines a modification of the optical signal based on the differential group delay and the principal states of polarization of the polarization mode dispersion. The PMD compensation system modifies the optical signal in the active optic fiber to compensate for PMD based on the determination of the modification. The PMD compensation system then transmits the optical signal.
In various embodiments of the invention, the PMD compensation system measures the differential group delay and the principal states of polarization of the PMD in the active optic fiber by estimating the differential group delay and the principal states of polarization of the PMD in the active optic fiber. The PMD compensation system modifies the optical signal by changing the polarization state of the optical signal. The PMD compensation system modifies the optical signal by changing the differential group delay of the PMD in the active optic fiber.
Advantageously, the invention adapts to the time varying nature of the PMD in the active optic fiber by measuring the differential group delay and the principal states of polarization. Also, the invention is applied to active optic fibers so the fiber optic communication system does not have to be taken out of operation to compensate for PMD. The invention advantageously scrambles a state of polarization of the optical signal to greatly improve the measurement of the differential group delay and the principal states of polarization.